1. Field of the Invention
This invention relates to a catalyst for use in direct oxidation fuel cells or other electrochemical reactor devices operating in the temperature range up to about 200.degree. C. More particularly, this invention relates to a single phase, high surface area catalyst comprising platinum, ruthenium and osmium.
2. Description of Prior Art
State-of-the-art catalysts for the electro-oxidation of hydrocarbon fuels such as methanol are based on platinum (Pt)-ruthenium (Ru) alloys. The overall electrochemical reaction for the oxidation of methanol on a fuel cell anode, the fuel electrode, is: EQU CH.sub.3 OH+H.sub.2 O.fwdarw.CO.sub.2 +6H.sup.+ +6e.sup.-
Platinum is a good catalyst for the adsorption and dehydrogenation of methanol molecules but it is rapidly poisoned as the catalyst surface becomes blocked with the adsorbed intermediate, carbon monoxide. It is known that H.sub.2 O plays a key role in the removal of such poisoning species from its role in the rate determining process of CO removal in accordance with the following reactions: EQU Pt+H.sub.2 O.fwdarw.Pt--OH+H.sup.+ +e.sup.- EQU Pt--CO+Pt--OH.fwdarw.2Pt+CO.sub.2 +H.sup.+ +e.sup.-
However, platinum does not adsorb H.sub.2 O species well at potentials where fuel electrodes operate in direct oxidation fuel cells. The result is that platinum is a poor catalyst for direct oxidation fuel cells and the steady-state activity necessary for fuel oxidation is poor.
Platinum-ruthenium, as previously stated, is presently the state-of-the-art catalyst for direct oxidation of methanol. The success of the platinum-ruthenium catalyst is based on the ability of ruthenium to adsorb H.sub.2 O species at potentials where methanol is adsorbing on the platinum and facilitate the carbon monoxide removal reaction. This dual function of the alloy catalyst's surface, that is, to adsorb both reactants on the catalyst surface on adjacent metal sites, is known as the bifunctional mechanism in accordance with the following reaction: EQU Pt--CO+Ru--OH.fwdarw.Pt+Ru+CO.sub.2 +H.sup.+ +e.sup.-
The combination of platinum and ruthenium adjacent metal sites forms an active site on the catalyst surface where methanol is oxidized in a non-poisoning way. The term "pair site" is used herein to describe the situation where adjacent metal atoms are adsorbing the methanol and water reactants. Appreciable rates of methanol oxidation are obtainable only in the presence of a high pair density. It follows that a single phase alloy crystal structure is desirable because the density of pair sites will be maximized on an alloy surface. The activity of the pair site, however, is the most important parameter. For example, several alloy surfaces with high pair site densities are known, but the activity for methanol oxidation is only appreciable on some of these surfaces. That is, if the catalytic activity of each pair site is low, then the resulting methanol oxidation will be poor. The elements that constitute the pair site are, thus, critical.
The best known catalysts for methanol oxidation are Pt--Ru, Pt--Sn, Pt--Mo, and Pt--Re. It is known that osmium (Os) adsorbs water at the most negative potential of all the noble metals, albeit still in the range of interest for fuel electrodes in direct oxidation fuel cells. As a result, osmium is a primary candidate for alloying with platinum to produce a good methanol oxidation catalyst. However, osmium possesses a maximum solubility of 20 at. % (atomic percentage) in platinum. Thus, while a pair site of Pt--Os may be intrinsically more active than a pair site of Pt--Ru, the limited solubility limits the pair site density to values smaller than that found in Pt--Ru. In addition, osmium can form OsO.sub.4, which is toxic to humans, under certain conditions.
U.S. Pat. No. 4,880,711 to Luczak et al. teaches a ternary alloy catalyst for fuel cells comprising platinum and gallium. Additional elements of the catalyst include chromium, cobalt, nickel and/or mixtures thereof. The alloy catalyst is indicated to require at least about 50% platinum to be an effective catalytic material; however, other elements in the same periodic group, namely iridium, rhodium, osmium and ruthenium are indicated to be substitutable for a portion of the platinum.
U.S. Pat. No. 4,127,468 to Alfenaar et al. teaches a process for producing metal electrodes in which a basis-metal electrode comprising a basis-metal which is present in a finely divided or porous state and which is selected from the group consisting of the noble metals from Groups IB, IIB, or VII of the Periodic Table of the Elements, or an alloy of at least one of said metals, is contacted with a solution containing an alloying element. The alloying element is selected from the group consisting of an element from Groups IIIA, IVA, VA, VIA, VII, IB, IIB, VIIB or combinations thereof, of the Periodic Table of the Elements. The alloying-element compound is reduced in situ to form a free-alloying element, whereby the alloying element forms an alloy with the basis-metal. Preferred basis-metals include palladium, platinum, palladium-platinum, and platinum-iridium.
U.S. Pat. No. 5,208,207 to Stonehart et al. teaches an electrocatalyst comprising an inorganic support and a ternary alloy essentially consisting of platinum-palladium-ruthenium supported on the support. U.S. Pat. No. 5,225,391 to Stonehart et al. teaches an electrocatalyst comprising an inorganic support and a four-element alloy consisting essentially of platinum, nickel, cobalt and manganese supported on the inorganic support. U.S. Pat. No. 5,286,580 to Ippommatsu et al. teaches a fuel electrode for a high temperature solid electrolyte fuel cell comprising ruthenium, osmium, rhodium or iridium or an alloy thereof.
None of the prior art of which we are aware teaches or suggests the importance of the crystal structure of the disclosed catalysts.